Field of the Invention
The present invention relates to a cathode material for a lithium-ion secondary battery, a cathode for a lithium-ion secondary battery, and a lithium-ion secondary battery.
Description of Related Art
In recent years, as batteries anticipated to have a small size and a high capacity and weigh less, non-aqueous electrolytic solution-based secondary batteries such as lithium-ion secondary batteries have been proposed and put into practical use. Lithium-ion secondary batteries are constituted of a cathode and an anode which have properties capable of reversibly intercalating and deintercalating lithium ions, and a non-aqueous electrolyte.
As anode active materials for anode materials of lithium-ion secondary batteries, generally, carbon-based materials or Li-containing metal oxides having properties capable of reversibly intercalating and deintercalating lithium ions are used. Examples of the Li-containing metal oxides include lithium titanate (Li4Ti5O12).
Meanwhile, as cathode materials of lithium-ion secondary batteries, cathode material mixtures including a cathode active material, a binder, and the like are used. As the cathode active material, for example, Li-containing metal oxides having properties capable of reversibly intercalating and deintercalating lithium ions such as lithium iron phosphate (LiFePO4) are used. In addition, cathodes of lithium-ion secondary batteries are formed by applying the cathode material mixture onto the surface of a metal foil that is called a current collector.
As electrolytic solutions for lithium-ion secondary batteries, non-aqueous solvents are used. Non-aqueous solvents enable the application of cathode active materials that oxidize and reduce at a high potential or anode active materials that oxidize and reduce at a low potential, whereby lithium-ion secondary batteries having a higher voltage can be realized.
These lithium-ion secondary batteries have a smaller size and a higher energy and weigh less than secondary batteries in the related art such as lead batteries, nickel cadmium batteries, and nickel metal hydride batteries. Therefore, lithium-ion secondary batteries are used not only as small-size power supplies used in portable electronic devices such as mobile phones and notebook personal computers but also as large-size stationary emergency power supplies.
In addition, recently, a broad range of studies have been continued regarding using lithium-ion secondary batteries for moving vehicles such as electrical vehicles, plug-in hybrid vehicles, and hybrid vehicles. For lithium-ion secondary batteries used for these moving vehicles, service lives of ten years or longer are required, and thus how to ensure long-term safety becomes important.
As cathode materials for lithium-ion secondary batteries having high safety, lithium phosphate compounds including lithium iron phosphate are well known. In these lithium phosphate compounds, since oxygen is strongly bonded to phosphorus by means of covalent bonding, it is difficult for oxygen to be released from crystals, and stability is excellent.
However, these lithium phosphate compounds have a problem of poor electron conductivity. Therefore, as a method for increasing the electron conductivity of cathode materials, for example, the following technique is known. The surfaces of the primary particles of a cathode active material are coated with an organic compound which is a carbon source, and then the organic compound is carbonized. In such a case, a conductive carbon film is formed on the surface of the cathode active material, and it is possible to interpose carbon in the conductive carbon film as an electron-conductive substance. A method for manufacturing a cathode material having increased electron conductivity in the above-described manner has been disclosed (for example, refer to Japanese Laid-open Patent Publication No. 2001-15111).
The effect of the conductive carbon film is extremely strong, and, in a case in which a lithium phosphate compound is used as a cathode material for lithium-ion secondary batteries, it has become common to form a conductive carbon film on the surfaces of the primary particles.